This invention relates to a decoder circuit, and particularly to a circuit for decoding telephone system coded ringing signals.
Multi-party line telephone system subscriber loops which have a plurality of subscriber's sets connected to the same subscriber's loop normally utilize some form of ringing encoding in order to alert the subscriber that his telephone set is to be answered, or different frequencies of ringing which are filtered at the telephone set and ring only a designated telephone. The present invention is concerned with the type of coded ringing in which series of long and short ring bursts are sent from the central office to all of the telephones connected to a particular subscriber's line. In such systems the subscriber, hearing a particular predesignated code which is unique to his station, recognizes his code and answers the telephone.
When this type of ringing code is sent from the central office along a particular multi-party subscriber's line, all of the telephones ring. It has been found to be annoying and distracting to the subscribers to listen to all of the ringing codes and to mentally decode them while only one of the parties to the multi-party line need answer the call. The attention and resulting mental stress which is required of all of the remaining parties could be eliminated if only the designated telephone were rung, and a subscriber need only listen for the presence of ringing, since he would know that the ringing which he hears is intended only for him.
The present invention is an automatic ringing decoder circuit which is intended to be used to decode ringing codes, thus relieving the subscribers of the stress of mentally decoding them. The invention can be used in conjunction with a switch in series with the portion of the subscriber's line leading connected to a particular subscriber's telephone, to switch the line to that subscriber only in the event the decoded ringing designates that the call is intended for that subscriber. The switch would remain closed unless ringing were received (facilitating outgoing calls), but as soon as ringing is received, the portion of the subscribers line leading to each of the subscribers would open until decoding has been effected. Following decoding, only the subscribers line leading to a particular subscriber would close, allowing further coded ringing signals to ring only the designated subscriber's telephone. That subscriber, hearing his telephone ring, merely answers it without mentally decoding the ringing.
The present invention is a circuit for effecting the ringing decoding. A copending Canadian patent application, Ser. No. 298,723, filed Mar. 13, 1978, entitled PARTYLINE SUBSCRIBER INTERFACE CIRCUIT and invented by Guido Panizzon et al describes ancillary apparatus which can be used in conjunction with the present invention to open and close the respective portions of the subscriber's line connected to the multi-party line, and is incorporated by reference.
Multi-party line systems require the use of revertive ringing, in which the central office is caused to ring all the telephones of a multi-partly line when one subscriber wishes to ring another party of the same multi-party line. This may consist of spaced short bursts of splash ringing.
The telephone switching office normally provides five different types of ringing codes as follows: C1 type comprising a long ring, C2 type comprising two long rings, C3 type comrpising a long ring followed by a short ring, C4 type comprising a long ring followed by two short rings, and C5 type comprising a long ring followed by a short ring, which is followed by a long ring. Ringing voltage, typically 90 volts a.c. at about 20 Hz is applied between the ring or tip lead and ground. Tip party subscriber sets are connected with reversed polariy, to facilitate decoding on ring or tip to ground.
It has been found that various types of central offices, while providing the above-described types of ringing, initiate the ringing bursts at different time sequences relative to each other, and indeed, provide long ring of different time periods. For example, the short ring bursts are standarized at 1/2 second each, but the long ring bursts can vary between 1 second and 11/2 seconds.
Further, the time tolerance of the ring bursts have been found to vary widely, and the transmission line characteristics of the subscribers lines, the longitudinal signal characteristics, the degree of imbalance, etc., all have been found to affect the characteristics of the ringing signal to a substantial degree. In addition, the time between sequences of ring bursts varies between the types of ring bursts noted above.
The above-noted difficulties have made machine decoding heretofore available not completely reliable, and consequently ringing decoding has generally been left to the human subscriber.
The present invention, on the other hand, provides means for reliable decoding of ringing signals, and further provides means for ignoring splash ringing.